In known image forming apparatuses, characters or image data are recorded as follows: a developer is supplied to an electrostatic latent image formed on a photosensitive member (photosensitive drum) to develop it into a visible image; and the visible image is transferred onto a recording medium. Of the known image forming apparatuses, a cartridge-type process unit as disclosed in, for example, JP 20000-250310, is employed in order to facilitate operations such as for maintenance and replacement.
The process unit (or process cartridge) is a photosensitive member cartridge having a photosensitive member (photosensitive drum) and a developer cartridge having a developing roller in an assembled condition. The developer cartridge is formed with an inverted triangular-shaped action-receiving portion, protruding from an outer surface of each side of the developer cartridge. When the developer cartridge is completely set in the photosensitive member cartridge, the action-receiving portion is restrained from above by a lower contact portion of a lock lever that is formed on a sidewall of the photosensitive member cartridge, so that the developer cartridge is prevented from becoming detached from the photosensitive member cartridge. In addition, the developer cartridge includes a toner containing chamber and an inlet through which toner is supplied to the toner containing chamber. The inlet is disposed so that it is exposed, when the developer cartridge is attached to the photosensitive member cartridge, from a sidewall of the developer cartridge opposite from the sidewall of the photosensitive member cartridge where the lock lever is formed.